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Old 26-05-2020, 10:11   #1
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Add second large alternator to charge house batteries

Is it possible to add a second larger alternators to twin Yanmar 3YM30, 29 HP, dedicated only to charging the house batteries? If yes, what is the largest alternator that is recommended?

Can such alternators be engaged and un-engaged at the helm station or electrical panel or navigation station.

Does having the large, second alternators partially or fully alleviate potential issues from idling the engine daily at a high enough RPM for the alternator to provide 50% of it's rated output due to the increased load of running the second laternator?

What are the pros and cons of this sort of set up as opposed to having a generator on a Cat that has two engines as described above to charge a large bank of lithium batteries. The boat has 1200 watts of solar and 800AH of lithium batteries at 12 volts.

As described above and with the size alternators you recommended how long would both engines need be idled to maintain output from both alternators of 50% of their rated value? is there a way to keep the alternators cool enough so that they don't lose output or at least don't lose significant output from getting hot or is it all about selecting the best possible alternator for the job?
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Old 26-05-2020, 10:18   #2
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Re: Add second large alternator to charge house batteries

If you're going to do that with the engine out of gear, you want to go as large as practical, but disconnect for normal use if possible. Running the engine with low loads will coke up the cylinders with unburnt fuel.
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Old 26-05-2020, 10:20   #3
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Re: Add second large alternator to charge house batteries

Might as well get a small petrol genny if you are unable to add more wind or solar power for some reason. Would cost about the same.
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Old 26-05-2020, 10:22   #4
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Re: Add second large alternator to charge house batteries

To me, a generator is more useful than a huge alternator if the choice is one or the other. Either one can charge batteries, but the generator can also heat hot water and handle other high power loads that you wouldn't want to run via inverter / solar.
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Old 26-05-2020, 10:58   #5
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Re: Add second large alternator to charge house batteries

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Originally Posted by rslifkin View Post
To me, a generator is more useful than a huge alternator if the choice is one or the other. Either one can charge batteries, but the generator can also heat hot water and handle other high power loads that you wouldn't want to run via inverter / solar.
Thanks for the feedback!


Maybe the benefit is not there from what I am asking about. The idea is to avoid having another engine on board since there are already two. Just seems potentially like a good idea.

I would rather not mess with a Honda 2200 which means propane or gas has to be used, and has to be used in a way to be safe. As long as the engines can handle the larger alternators while motoring then in more instances there would be no need to use a portable generator first thing in the morning or at the end of the day.

The idea is to have enough lithium so when the batteries are fully charged the batteries can power a 12,000 BTU air conditioner 12 hours mostly at night.

So the question is can I avoid having to have a small built in generator or running a portable generator frequently by having two larger, second alternators on each engine. The two larger alternators only work if they can used while motoring and when necessary at a higher RPM when idling without harming the engine. If the above works then I avoid paying for an expensive built in noisy, expensive
generator or using the portable while being anchored at the end of the day when the noise and hassle is a negative.
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Old 26-05-2020, 11:17   #6
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Re: Add second large alternator to charge house batteries

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The idea is to have enough lithium so when the batteries are fully charged the batteries can power a 12,000 BTU air conditioner 12 hours mostly at night.
I realise the Caribbean is hot, but you can't be talked into just increasing your interior air circulation instead, with a windcatcher and fans?
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Old 26-05-2020, 11:51   #7
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Re: Add second large alternator to charge house batteries

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I realise the Caribbean is hot, but you can't be talked into just increasing your interior air circulation instead, with a windcatcher and fans?
My need is for Rio Dulce, Guatemala in summer which is 20 miles inland from the Caribbean Sea. Anywhere anchored in the Caribbean Sea I can get by 12 months year with fans, wind scoop, etc., no air conditioning. It that environment I prefer no air conditioning unless there is no breeze which rarely occurs.
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Old 26-05-2020, 11:54   #8
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Re: Add second large alternator to charge house batteries

Ouch, yea, I imagine that can get sticky...
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Old 26-05-2020, 13:01   #9
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Re: Add second large alternator to charge house batteries

I have been researching this exact circumstance for my self. I donít have a generator on my catamaran. I do have a Honda generator for back up for only power for mandatory electronics while under sail, and can power my Cruise RO water maker. The Honda doesnít produce optimal amperage to charge lithium batteries, like a 200amp large frame alternator. When no wind and motoring, you get the power in addition to getting somewhere. At anchor, would take considerable less run time to help solar, then a factory 80amp, that can only push about 40 amps continuously, if lucky. The 200amp works out to about 1/4 ratio runtime, 200amp 1 hour equals 4 hours with a 80amp.

My system should supply most of my power demands without needing alternate charging supplied. The alternator mostly as backup or harvesting the power while already motor for lack of wind, ie be able to run AC or water maker, etc. from the additional power gain.

The argument of no load on motor at anchor to charge the batteries is bad on the motor, yes. Thats why itís better to have the largest alternator charging amperage, if doing so. Cuts down on run time, large alternator pushing 180amps, will put more demand on the motor to drive the alternator, putting more load on the engine.
Additional benefits are: weight and storage space saved from a heavy bulky generator, cost for me having to buy a 9K generator and having it installed, maintenance of another motor, no noise of a generator at anchor.
My system will be 2k rated worth of solar panels and 1000Ah of lithium batteries. Plenty of power produced and stored for my needs for the areas I plan to cruise, latitudes near the equator. So, this is the best solution for my situation, and maybe not the solution for others. The cost of my power supply system cost about the same as buying and having a 9K generator installed.
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Old 26-05-2020, 13:26   #10
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Re: Add second large alternator to charge house batteries

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Originally Posted by 69Datsun510 View Post
I have been researching this exact circumstance for my self. I donít have a generator on my catamaran. I do have a Honda generator for back up for only power for mandatory electronics while under sail, and can power my Cruise RO water maker. The Honda doesnít produce optimal amperage to charge lithium batteries, like a 200amp large frame alternator. When no wind and motoring, you get the power in addition to getting somewhere. At anchor, would take considerable less run time to help solar, then a factory 80amp, that can only push about 40 amps continuously, if lucky. The 200amp works out to about 1/4 ratio runtime, 200amp 1 hour equals 4 hours with a 80amp.

My system should supply most of my power demands without needing alternate charging supplied. The alternator mostly as backup or harvesting the power while already motor for lack of wind, ie be able to run AC or water maker, etc. from the additional power gain.

The argument of no load on motor at anchor to charge the batteries is bad on the motor, yes. Thats why itís better to have the largest alternator charging amperage, if doing so. Cuts down on run time, large alternator pushing 180amps, will put more demand on the motor to drive the alternator, putting more load on the engine.
Additional benefits are: weight and storage space saved from a heavy bulky generator, cost for me having to buy a 9K generator and having it installed, maintenance of another motor, no noise of a generator at anchor.
My system will be 2k rated worth of solar panels and 1000Ah of lithium batteries. Plenty of power produced and stored for my needs for the areas I plan to cruise, latitudes near the equator. So, this is the best solution for my situation, and maybe not the solution for others. The cost of my power supply system cost about the same as buying and having a 9K generator installed.
How much horsepower can your engines provide? Have you checked with the manufacturer of your engines to see what the max size second alternator they recommend can be used? Are you planning to install the same second alternator on both engines? Are you planning to be able to engage and disengage the second large alternator on each engine at the helm station, the nav table or breaker panel?

Some say idling marine diesel engines to charge batteries is not harmful as long as the engines get run under load at cruising speed RPM every couple of days for 1-2 hours. It is expensive in the sense that hours are being put on the engines. If worst case you have to run the engines an hour or so in neutral at approx. 1500 RPM to fully charge lithium batteries that are at 20% charged it is worth it to not have a generator and all that goes with it: upfront cost, cost to maintain, weight.

A Honda portable generator is always good to have. But I don't want to use except as a back up.

I want to go electric cook top and electric oven. Only use propane for the grill outside. That way if no electricity can cook outside and if no propane can cook inside.
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Old 26-05-2020, 14:25   #11
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Re: Add second large alternator to charge house batteries

Quote:
Originally Posted by Augi View Post
Is it possible to add a second larger alternators to twin Yanmar 3YM30, 29 HP, dedicated only to charging the house batteries? If yes, what is the largest alternator that is recommended?

The usual limiting problems are finding room for them and for the belt drive components. For very large alternators, figuring out a reliable, vibration-free way to mount them is a problem, and concerns arise regarding the side loads that the belt places on the front main bearing for the engine. What is possible varies from boat to boat.


There aren't very many installations on smaller Yanmars that produce more than 200 amps at 12v.


Quote:

Can such alternators be engaged and un-engaged at the helm station or electrical panel or navigation station.
Yes, by shutting off or reducing the power to the field windings. Most modern external regulars are designed to do this.


Quote:

Does having the large, second alternators partially or fully alleviate potential issues from idling the engine daily at a high enough RPM for the alternator to provide 50% of it's rated output due to the increased load of running the second laternator?
Depends who you believe and how often and how long you plan on doing it.


Quote:

What are the pros and cons of this sort of set up as opposed to having a generator on a Cat that has two engines as described above to charge a large bank of lithium batteries. The boat has 1200 watts of solar and 800AH of lithium batteries at 12 volts.
Generator sets are heavy, expensive, and difficult to install well. The quiet ones are heavier and more expensive than the loud ones. Over time they require maintenance and periodic replacement even if lightly used. On the other hand they produce more power all at once so you potentially could have shorter run times.



Using the main engines to make electricity while stationary puts extra hours on them and hastens the day when they will require replacement.


Quote:

As described above and with the size alternators you recommended how long would both engines need be idled to maintain output from both alternators of 50% of their rated value? is there a way to keep the alternators cool enough so that they don't lose output or at least don't lose significant output from getting hot or is it all about selecting the best possible alternator for the job?
The most common approach is to replace the stock alternator with an overwound one that is the same physical size which limits you to about 120 amps, and then adding provisions so that this new alternator can charge both the house battery and engine start battery.


Anything beyond that becomes a science project, an experiment, that requires custom brackets, modifications to the engine enclosure, and the possibility of custom cooling and belt routing.
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Old 26-05-2020, 14:44   #12
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Re: Add second large alternator to charge house batteries

If you want to do this on a Yanmar, you need to read the installation manual for your particular motor, for motors much larger than the ones you are taking about, many are limited to 4 HP at 1800 RPM and 8HP at wide open throttle.
That means to extract 100 amps from my 44 HP Yanmar and an over 100 HP turbo Yanmar we can pull 100 amps max, but have to run the motors at 1800 RPM to do so, 50 amps at 900 RPM, and 200 at redline. A Honda can easily make 100+ amps.
This is a physical engine limitation on how much power can be extracted from the front of the crankshaft without damaging the engine. It’s not belt tightness etc, it’s how much torque you can harvest from the front of the crank so having two alternators opposite each other won’t help.
Don’t believe me, that’s fine. Get a copy of the installation manual for your specific model and see what it says, what my limits are may have nothing to do with yours.

On edit, I have yet to see a non exotic small frame alternator make more than 100 amps continuously without running way hotter than I’m comfortable with, by non exotics I mean those without remote mounted diodes etc, so it’s kind of a mute point, people buy 200 amp alternators all the time and what saved them from being burnt up is temp monitoring and power cut back, and or most likely that a lead acid battery bank won’t take that kind of power for more than a minute or two anyway, then it’s acceptance rate drops and the alternator doesn’t burn up.
Of course Lithium’s can and apparently do burn up alternators, unless they are seriously derated to way less
than their marketing output number.

So to get even 100 amps you need to run your motor at 1800 RPM (depending on your motor of course) and besides even if it weren’t an engine limit to get 100 amps continuously you probably have to spin the motor that fast to spin the alternator fast enough so that it’s fans can keep it cool.

A Honda’s $1,000, is very reliable, puts no hours on your motor and you don’t have to modify anything.
Go price a big alternator, the external regulator you need to run it, and then the serpentine belt kit you need for it and your in for way more money than a Honda.
I believe all in it’s about $2,000 roughly
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Old 26-05-2020, 15:14   #13
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Re: Add second large alternator to charge house batteries

Quote:
Originally Posted by Jammer View Post
The usual limiting problems are finding room for them and for the belt drive components. For very large alternators, figuring out a reliable, vibration-free way to mount them is a problem, and concerns arise regarding the side loads that the belt places on the front main bearing for the engine. What is possible varies from boat to boat.


There aren't very many installations on smaller Yanmars that produce more than 200 amps at 12v.



Yes, by shutting off or reducing the power to the field windings. Most modern external regulars are designed to do this.



Depends who you believe and how often and how long you plan on doing it.



Generator sets are heavy, expensive, and difficult to install well. The quiet ones are heavier and more expensive than the loud ones. Over time they require maintenance and periodic replacement even if lightly used. On the other hand they produce more power all at once so you potentially could have shorter run times.



Using the main engines to make electricity while stationary puts extra hours on them and hastens the day when they will require replacement.



The most common approach is to replace the stock alternator with an overwound one that is the same physical size which limits you to about 120 amps, and then adding provisions so that this new alternator can charge both the house battery and engine start battery.


Anything beyond that becomes a science project, an experiment, that requires custom brackets, modifications to the engine enclosure, and the possibility of custom cooling and belt routing.
Thanks for the feed back. Very helpful.
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Old 26-05-2020, 16:41   #14
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Re: Add second large alternator to charge house batteries

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Originally Posted by Augi View Post
How much horsepower can your engines provide? Have you checked with the manufacturer of your engines to see what the max size second alternator they recommend can be used? Are you planning to install the same second alternator on both engines? Are you planning to be able to engage and disengage the second large alternator on each engine at the helm station, the nav table or breaker panel?

Some say idling marine diesel engines to charge batteries is not harmful as long as the engines get run under load at cruising speed RPM every couple of days for 1-2 hours. It is expensive in the sense that hours are being put on the engines. If worst case you have to run the engines an hour or so in neutral at approx. 1500 RPM to fully charge lithium batteries that are at 20% charged it is worth it to not have a generator and all that goes with it: upfront cost, cost to maintain, weight.

A Honda portable generator is always good to have. But I don't want to use except as a back up.

I want to go electric cook top and electric oven. Only use propane for the grill outside. That way if no electricity can cook outside and if no propane can cook inside.
In my set up, Iím suggesting to only have one alternator per engine, one a large framed 200amp, and on stock. To install a large frame 200-250amp alternator, will require a serpentine belt upgrade, too. I would only use the large alternator, as secondary charging method, if the solar wasnít able to keep up over days of low output. Or if motoring, harvest power on a motor already being used for propulsion.
Only use the large alternator on anchorage as a backup or emergency charge because batteries are getting low.

The Honda would be to power the Cruise RO water maker, if I needed to conserve power, or in an emergency to have power for navigation equipment during a passage, should the solar or lithium battery system stopped working. I would have essential power while getting the main system back up and running.

For data on such system, read CatNewBeeís build on solar ad lithium, and well appointed all electric galley. He has been using his system for 3 years now, I think. https://www.cruisersforum.com/forums...ey-201795.html

My thoughts are, I have a sailboat, want to sail, and not rely on motors, boat or generator. Therefore, I will have a system design with enough solar and lithium to be comfortable. We are fortunate with owning catamarans, we can put up large solar panel arrays, have areas to put large lithium battery packs. The technology of these components has made it possible to do suggest task
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Old 26-05-2020, 17:02   #15
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Re: Add second large alternator to charge house batteries

Yanmar is pretty good about providing details for this. The summary is this picture.

Click image for larger version

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The details for the 3YM30 are in the installation manual.

You can mostly unload the engine when the alternator is not needed by using an external regulator and interrupting the field current. This is pretty straightforward and just leaves the spinning friction load. You might be able to unload slightly more by using a clutch on the alternator pulley, but you won't gain much (an unquantified value) other than complexity compared with just turning off the field.

Note: remember that your alternator is probably only ~50% efficient, the values in the graph are the load on the engine, not the power output at the alternator. Figure maximum power output at alternator = about one-half the value on the graph. At 2000 engine RPM that leaves about 85A output. So, maybe a 100A alternator, but it might need a load manager in the regulator (starting to get into specifics that would depend on pulley ratio, mounting conditions as compared to other overhung equipment, ...)
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